In many applications utilizing a power amplifier, it is desirable to produce an amplitude modulated output. For instance, in Enhanced Data for GSM Evolution (“EDGE”) cellular phones, the power amplifier may be required to produce an amplitude modulated signal during a data transmission burst. Other systems can also have this requirement, such as code-division multiple-access (CDMA), wideband CDMA (WCDMA), 802.11, and other transmission systems.
FIG. 1 is a diagram 100 of a representative plot of the output power of a prior art power amplifier in a time-division multiple-access communications system using amplitude modulation, such as in EDGE cellular telephony. The output power of the power amplifier, indicated by the representative curve 101, may be controlled from a low level before transmitting data, modulated about a higher level when transmitting data in data transmission burst region 103, and then brought down to a low level after the data is sent. Many systems have requirements that the power must be held between certain levels, indicated by the power mask 102, so that the power is held within specified limits at all times during the transmission. It is also common that the output frequency spectrum has limits placed on it so that the particular shape of the ramp up, ramp down, and modulation may need to be accurately controlled. If the shape deviates from the desired shape, the output frequency spectrum may fail these limits. Additionally, the data transmission burst region 103 can have strict requirements on signal distortion in this region in order to ensure that the transmitted data can be recovered by a receiver or to avoid corruption of data being transmitted between other devices in a nearby channel. It is also often desirable to have accurate control over the output power within data transmission burst region 103. Each of these requirements may have to be met by the transmitter system.
One way in which signal modulation may be accomplished is by utilizing a polar transmission loop. In a polar transmission loop, the desired transmit signal can be decomposed into an amplitude modulation component and a phase modulation component. The amplitude modulation component can be then produced using a power amplifier whose output power can be controlled, such as by using a variable gain power amplifier or a power control loop. The phase modulation component is typically produced by providing the input to the power amplifier as an appropriately phase-modulated signal.
FIG. 2 is a diagram of a prior art system 200 that provides power amplifier and polar modulation using output voltage detection. Although open-loop polar modulation can be used, closed-loop polar modulation can be advantageous when modulation accuracy is desired. System 200 includes power amplifier 212, radio frequency (RF) amplitude detector 215, error amplifier 217, modulation control circuit 203, and phase modulator 204. Modulation control circuit 203 receives modulation signal 202 and produces an amplitude modulation control signal 218 and a phase modulation control signal 205. Modulation signal 202 can be quadrature control signals such as I/Q signals, data streams such as bits to be transmitted, or other suitable signals. Phase modulation control signal 205 is used to produce phase modulated RF signal 213, such as by using phase modulator 204 and unmodulated RF source signal 201. Alternately, phase modulated RF signal 213 can be produced from phase modulation control signal 205 using a phase locked loop (PLL), a modulator employing mixers, or in another suitable manner.
Power amplifier 212 receives phase modulated RF signal 213 as an input, which it amplifies to produce RF output 214. The amplitude of RF output 214 can be adjusted by control signal 219. RF amplitude detector 215 generates a feedback signal 216 related to the sensed amplitude of RF output 214. Error amplifier 217, which can be an integrating amplifier or other suitable differencing amplifiers, compares feedback signal 216 to amplitude modulation control signal 218 so as to adjust control signal 219 to reduce the difference between the feedback and modulation control signals. In this manner, amplitude modulation control signal 218 can control the output power or amplitude of the power amplifier. Other types of detectors can be also or alternatively be used to generate feedback signal 216, such as a detector sensing the output RF current, a detector sensing the power from a directional coupler, or other suitable detectors or circuits.
An issue that can arise in systems using closed loop polar feedback is that amplitude modulation control signal 218 can be sufficiently high that power amplifier 212 is not capable of producing the requested output power. This can occur, for instance, if the power amplifier is presented with a load mismatch so that under this mismatch the power output capability of the power amplifier is reduced. This can also occur under a load mismatch if RF amplitude detector 215 incorrectly estimates that the output power of power amplifier 212 is lower than the actual output power, causing polar modulation loop to attempt to produce a higher output power than actually required.
If the power amplifier is incapable of generating the output power level that is required in response to control signal 219, the output power can be less than the requested power for the duration of time wherein the power requested is higher than can be made. Because the loop can only produce the power amplifier's maximum output power, the output signal can result in clipped-off peaks in the output level which are higher than the maximum power that the power amplifier can produce. This clipping of the output power can be disadvantageous in multiple ways, as discussed below.
FIG. 3 is a diagram 300 of a representative prior art power versus time plot in a situation where clipping occurs. Waveform 301 depicts the output power of power amplifier 212 across a transmission time slot in a typical Time Division Multiple Access (TDMA) system such as EDGE. In this case, requested output power 304 is at times higher than the maximum power that the amplifier can produce, as indicated by line 302. At the times when the amplifier is requested to make more power than this maximum power, the amplifier will typically instead produce its maximum power such as is indicated by several flattened-top regions 305.
This response can cause a failure to pass the power mask 102 of the time mask, as the power waveform in the region around area 308 can be above the mask. Additionally, a sharp corner in the power versus time plot such as area 309 can cause failure to comply with the output frequency spectrum. Signal distortions in the data transmission burst region 103 can also cause failure to comply with output modulation spectrum during the data burst, which can be have even tighter limits. Furthermore, signal clipping in data transmission burst region 103 can distort the transmitted output signal, potentially degrading the ability of the receiver to recover the transmitted signal correctly. Measures of signal quality such as error vector magnitude (EVM) and bit error rate (BER) can be adversely impacted by this signal clipping.
Another issue that can arise in systems using closed loop polar feedback is that RF output 214 can, under certain circumstances, be sufficiently low that RF amplitude detector 215 is not able to correctly detect the amplitude of RF output 214. This condition can occur, for instance, if the amplitude modulation control signal 218 is at a sufficiently low level such as if the desired transmit power is low. In many applications, such as cellular telephony, it can be necessary to transmit at several different power levels. When transmitting at the lower power levels, RF amplitude detector 215 may not receive a large enough signal to correctly detect the transmitted amplitude.
FIG. 4 a diagram 400 of an input to output relationship of a prior art RF detector such as may be used to implement RF amplitude detector 215. Solid line waveform 401 depicts feedback signal 216 of RF amplitude detector 215 versus the amplitude of RF output 214 applied to the detector's input. A prior art detector may exploit a nonlinearity of a device inside of the detector and so may require a sufficiently large input signal in order to activate that nonlinearity and respond to further changes in RF input level. This nonlinearity can result in an effective input offset amplitude 402, so that a linear extrapolation of the input-output response of waveform 401 diverges away from the actual response at low input amplitudes. Distortion of input-output response of waveform 401 at low amplitudes can be a result of other problems in prior art systems, such as crossover distortion, transistor mismatch, and other mechanisms. As a result of this distortion at low input amplitude, RF amplitude detector 215 may provide useful feedback only over a range of sufficiently large signal amplitudes, such as range 403, and may fail to meet system requirements at low signal amplitude levels.
FIG. 5 is a diagram 500 of a representative prior art power versus time plot where a prior art RF amplitude detector 215 is not receiving sufficient signal amplitude. Waveform 501 depicts a desired output power of power amplifier 212 across a transmission time slot in a typical TDMA system such as EDGE. In this case, the desired output power of waveform 501 is at times lower than the minimum power that RF amplitude detector 215 can accurately detect. At the times when the amplifier is requested to make less power than this minimum power, the amplifier will typically instead produce an output power depicted by waveform 502, which is different from the desired output power of waveform 501. For instance, if RF amplitude detector 215 has a response similar to the one depicted in FIG. 4, the output power can be higher than desired since the detector can interpret the output power as being lower than it actually is.
This distortion can cause failure to comply with output modulation spectrum limits. Furthermore, if such signal distortion occurs in data transmission burst region 103, the ability of a receiver to recover the transmitted signal correctly can be degraded. Measures of signal quality such as error vector magnitude (EVM) and bit error rate (BER) can be adversely impacted by this signal distortion.